Engines for automobiles produce various vibrations having a wide variety of amplitudes and frequencies depending upon engine speeds. Various types of engine mounts have been developed to absorb or reduce the vibrations of the engines. One example of the engine mounts is disclosed in Japanese Patent Laid-Open Publication No. 2001-27279 entitled “LIQUID-FILLED ENGINE MOUNT”.
The disclosed engine mount includes a rubber member positioned closely to an engine, and a diaphragm positioned in opposed relation to the rubber member. The rubber member and the diaphragm jointly define a space. Such a space is separated by a partition member into primary and secondary chambers filled with liquid. The partition member has first and second orifices provided therein. The first orifice is always opened, whereby the primary chamber communicates with the secondary chamber. The second orifice includes a connection port positioned at a lower end thereof. The diaphragm is urged by a return spring to close the connection port. The engine mount includes a valve for opening/closing the second orifice, and a switch valve. When these valves are actuated, the diaphragm is forced to open the second orifice against a resilient force the return spring exerts on the diaphragm.
When started,the engine runs at an idling speed. During the idling of the engine, the switch valve is actuated such that the diaphragm is lowered by a negative pressure produced within an intake manifold of the engine to thereby open the second orifice.
When the engine idles with the second orifice opened, the liquid flows between the primary and secondary chambers by passing through the first and second orifices, and the rubber member elastically deforms to thereby reduce vibration of the engine.
On the other hand, when the engine runs at an engine speed higher than the idling speed, the second orifice is closed. The liquid fails to flow through the second orifice. Thus, the vibration of the engine is reduced by the deformation of the rubber member and the flow of the liquid only through the first orifice.
For the engine mount thus discussed, the diaphragm and the partition member have their flat surfaces held in contact with each other to close the connection port. In closing the connection port, more specifically, the return spring forces the flat surface of the diaphragm into violent impact on the flat surface of the partition member.
At a time when the engine is started with the connection port closed by the diaphragm, little or no negative pressure required to lower the diaphragm is produced within the intake manifold. It is thus impossible that the diaphragm is lowered using the negative pressure so as to open the second orifice.
At the same time, the rubber member intermittently undergoes a relatively large compressing force produced under the influence of cranking and the like. It takes only a short period of time per application of such a compressing force to the rubber member. In response to each application of the compressing force to the rubber member, the liquid within the primary chamber exerts a pressure on the diaphragm in such a manner as to flow out of the second orifice. The compressing force is large, and hence the pressure exerted on the diaphragm by the liquid within the primary fluid chamber and the first orifice is large, too. When the pressure exerted on the diaphragm by the liquid is greater than the resilient force exerted on the diaphragm by the return spring, the diaphragm moves away from the surface of the partition member by a very small distance to thereby open the connection port.
Each time the diaphragm moves away from the surface of the partition member, the application of the compressing force to the rubber member is terminated. This allows the return spring to force the surface of the diaphragm back into violent impact on the surface of the partition member. The impact between the surfaces of the diaphragm and the partition member produces a sound or noise which is potentially transmitted into a passenger compartment of an automobile. Such a noisy sound should be reduced.